This invention relates to characterizing semiconductor structures.
Shortly after the commercial development of the transistor and other structures built on semiconductor substrates, a four-point probe was introduced to measure the sheet resistance of doped semiconductor layers. (See for example, L. B. Valdes, Proc. IRE, Volume 42, 1954, or in A.S.T.M. Standard F374, Annual Book of A.S.T.M. Standards Vol. 10.05.) To measure dopant concentration as a function of depth in a substrate, layers were successively removed from the substrate by abrasive lapping or chemical etching. After removing each layer at a processing station, the substrate was transferred to a measurement station to measure the sheet resistance. As layers of known thickness are removed, the sheet resistance changes. From these changes, a resistivity profile can be calculated, and converted into a dopant concentration distribution based on an assumption of mobility values which provide a relationship between resistivity and dopant concentration.
Layers can also be removed from a silicon substrate by anodic oxidation that allows removing layers parallel to the surface, as reported by Eileen Tannenbaum of Bell Telephone Labs (Solid-State Electronics Vol. 2, 1961). Tannenbaum's technique includes growing an anodic oxide layer, stripping off the oxide layer with hydrofluoric acid to expose the bare silicon, and measuring the sheet resistance on this fresh bare surface with a four-point probe. Tannenbaum analyzed junction depths that were several microns deep, and each removed layer had a thickness of about 400 angstroms. Thus a single analysis required about fifty or more removing steps, and each step took about thirty minutes. Tannenbaum notes that when anodization has proceeded to within 2000 angstroms of the junction, where the sheet resistance is about 1000 ohm/square, the four-point probe measurements become imprecise. Shallow junctions that are less than 300 angstroms deep typically cannot be evaluated by Tannenbaum's technique.
In 1970, the technique of Hall effect measurement, described in detail in ASTM standard F76, Annual Book of ASTM Standards, vol. 10.05, was combined with the Tannenbaum anodic oxidation stripping technique (N. G. E. Johansson, J. W. Mayer, and D. J. Marsh, Solid-State Electronics vol. 13, 1970). A van der Pauw test resistor was etched out on the surface of a blanket implanted silicon wafer. The test station was a Hall-effect measurement apparatus which measured the sheet resistance under three conditions: with a positive magnetic field, an equal negative magnetic field and without any magnetic field. These measurements permit obtaining the resistivity profile and the mobility profile, and thus the dopant concentration distribution. However, this differential Hall effect (DHE) technique was subject to the time and labor limitations of the Tannebaum anodic oxidation stripping technique.
One attempt to automate the Hall effect measurement is described in R. Galloni and A. Sardo, Rev. Sci. Instrum., vol. 54, 1983. In this technique, a cloverleaf pattern is etched on a square of silicon that is mounted on a fiber-glass board holder and lowered into an anodizing pot where an oxide of predetermined thickness is grown. This specimen is raised from the anodizing pot and lowered successively into three additional pots, where it is washed, the anodic oxide is stripped with an HF solution, and the specimen is rinsed, respectively. After drying, the specimen is moved to an area where electrical measurements are made and the process is repeated. The authors mention a time span of three hours for the completion of their test.
Another technique, spreading resistance analysis (SRA), introduced by R. G. Mazur and D. H. Dickey, J. Electrochem. Society Vol. 113, page 255, 1966), uses a small piece of silicon that is angle-lapped, and its resistance is measured along the beveled surface. Because an entire profile can be evaluated in about thirty minutes using SRA, it became the standard method for generating resistivity profiles, as described in detail in A.S.T.M. standard F672 in the Annual Book Of ASTM Standards Vol. 10.05. SRA is typically not used for junctions having a depth of about 300 angstroms or less.
During the 1980's, secondary ion spectroscopy (SIMS) was adapted for dopant profile evaluation. This technique evaluates the dopant concentration distribution directly by sputter depth probing of a small piece of silicon that is broken out of the wafer and placed in the SIMS chamber. This technology requires a high cost equipment and highly trained operators. Thus these tests are carried out primarily by a small number of dedicated laboratories.
Other techniques for resisitivity profiling have also been proposed. These proposed techniques include scanning capacitance microscopy, nano capacitance-voltage method, scanning tunneling microscopy or the carrier illumination technique.